A hydraulic shock absorber, for example, as described in Patent Document, Japanese Patent Application Laid-Open No. 2006-250309, has a configuration in which a piston chamber of a cylinder tube is filled with oil such as mineral oil and a braking piston is accommodated in the piston chamber in a state that a communication gap of the oil is kept at an outer periphery of the piston, and a rod connected to the piston is extended outward from the cylinder tube. Then, kinetic energy of the moving object is absorbed by flow resistance of the oil flowing through the communication gap when the piston shifts by collision of the moving object at the leading end of the rod.
The rod, which is always urged by a restoration spring toward an initial position, repeats an operation of advancing to the initial position by energizing force of the restoration spring after the moving object collides and is retracted to a cushioning position. At this time, since an oil film is formed on the surface of the rod by adhesion of the oil filling the piston chamber, the oil in the piston chamber flows toward outside little by little via the rod caused by repetition of backward and forward movements. As a result, generally, shock-absorbing capacity as the shock absorber lowers while the oil amount in the piston chamber is gradually reduced, and thus the hydraulic shock absorber comes to the end of its life.
In order to prevent lowering of the shock-absorbing capacity caused by such a reduction of the oil amount, the shock absorber may be constituted, for example, as shown in FIG. 3. More specifically, separately from a piston chamber 23 in which the piston 21 and the oil 22 are accommodated, a reservoir tank 24 communicating to the piston chamber 23 is provided in the inside of the cylinder tube 20. The oil 22 in the reservoir tank 24 is always pressurized with a spring 26 via a pressure piston 25. Even though the oil in the piston chamber flows to the outside with the rod 27, the oil amount in the piston chamber 23 is always maintained constant in such a way that the oil corresponding to the reduction amount is replenished from the reservoir tank 24.
However, the shock absorber having such a configuration is designed to pressurize the oil in the reservoir tank 24 with the spring 26 via the pressure piston 25. For this reason, not only the configuration for pressure-application is complicated, but also a wide arrangement space for arranging the pressure piston 25 and the spring 26 becomes necessary, which poses a problem in that an axial length of the shock absorber becomes large.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-250309